import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentMap {
    public static void main(String[] args) {
        ConcurrentHashMap<Object, Object> map = new ConcurrentHashMap<>();
        //  map.put("a", "123");
        System.out.println(map);
        Object a = map.putIfAbsent("b", "1234");
        System.out.println(a);
    }
}
